1. Field of the Invention
The present invention relates to turbochargers having valves operable to cause the exhaust gases to by-pass the turbocharger.
2. Description of the Related Art
It is known in the field of internal combustion engines that catalytic converters, which are extensively used in the automobile industry to reduce engine emissions, operate with significantly reduced efficiency during cold start and engine warm-up conditions, resulting in higher than normal exhaust emissions during these periods. It is therefore recognised that thermal inertia within the exhaust system of such an engine should be kept to a minimum during cold start conditions in order to maximise the heat delivered to the catalytic converter from the engine, thereby promoting light off of the catalyst and so minimising the time during which emissions are at an unacceptable level.
Turbochargers are now commonly used to increase the power delivered by an engine by utilising the waste energy of the exhaust gases to power a turbine which compresses the intake air delivered to the combustion chamber. However, the provision of a turbocharger in the exhaust system of the engine imposes extra thermal inertia on the system, which delays catalytic converter light off in such engines and so increasing the time during which engine emissions exceed an acceptable level. In order to overcome this problem, turbochargers have been developed which have two separate ducts at the turbine inlet flange, one duct directing the exhaust gas to the turbine volute and the other providing a bypass for the turbine volute. A valve is provided in each duct to control the flow of exhaust gases therethrough. During normal engine operating conditions, the valve in the turbine volute duct is fully open, allowing exhaust gas to power the turbocharger, whilst the valve in the bypass duct operates to vary the through-flow in the bypass duct so as to provide standard wastegate control of the pressure in the turbine volute in order to prevent over-pressuring. During cold start conditions, however, the turbine volute valve is closed and the bypass valve opened so that the exhaust gases bypass the turbine and are instead directed to the catalytic converter with minimum heat loss, thereby promoting rapid light-off of the catalyst. Conventional such systems have the problem, however, that they require two separate actuators, one for each valve, due to the different range and sequence of movement required for each valve, and this increases the complexity and hence cost of the turbocharger system as well as increasing its overall size.
There is hence a need for a turbocharger which includes a system to enable exhaust gases to bypass the turbine during cold start conditions and instead to be fed to the catalyst with minimal heat loss so as to promote catalytic converter light-off, whilst being of reduced size and complexity as compared with existing turbochargers.
According to the present invention, there is provided a turbocharger assembly comprising a turbine bypass duct having a first valve disposed therein, and a turbine feed duct having a second valve disposed therein, characterised in that the first and second valves are coupled to a link member, the first valve being operatively attached to said link member so as to be constrained for movement therewith throughout the full range of operative movement of the link member, and the second valve having a lost motion coupling with the link member so as to be constrained for movement therewith through only a part of the full range of operative movement of the link member , wherein upon movement of said link member between a start position and an intermediate position, operative movement of the first valve only is effected and upon movement of the link member between said intermediate position and an end position, operative movement of the first and second valves occurs.
A turbocharger assembly in accordance with the invention has the advantage that it requires just a single actuator to control the valves in the turbine feed and turbine bypass ducts so as to provide wastegate operation during normal operating conditions of the turbocharger in order to prevent over-pressuring, as well as exhaust gas bypass of the turbine during cold start conditions in order to promote rapid catalytic converter light-off using, thereby reducing cost. It also has the advantage that the system is much more compact since space is required for just a single actuator.
In a preferred embodiment of the invention, the lost motion coupling between the second valve and the link member is effected by means of an elongated channel formed in the link member in which an actuating pin of the second valve engages. During movement of the link member between its start position and the intermediate position, the actuating pin of the second valve travels along the channel without moving relative to the turbine volute inlet duct, and upon movement of the link member from its intermediate position to its end position, the actuating pin moves with the link member through engagement with a surface of the channel, moving the actuating pin relative to the turbine volute inlet duct and hence closing the second valve.
The elongated channel is advantageously formed with two distinct sections, a first section along which the actuating pin of the second valve moves upon movement of the link member between the start position and the intermediate position, and a second section which imparts a camming action to the actuating member of the second valve as it moves therealong upon movement of the link member between said intermediate position and the end position. By suitable choice of the shape of the first and second sections, not only can movement of the second valve be eliminated during movement of the link member between said start position and said intermediate position, but also the speed and sensitivity of the movement of the second valve during movement of the link member between said intermediate and end positions can also be controlled.
In one embodiment of the invention, the link member is mounted for pivotal movement in the turbocharger assembly and the second valve is a rotary valve. The elongated channel is then formed such that the first section subtends an arc centred on the axis of rotation of the link member and the second section subtends an arc centred on a point offset from the axis of rotation of the link member, the offset of the centre of the arc of the second section from the axis of rotation of the link member and the radius of curvature of the second arc being used to vary the sensitivity of the second valve to the movement of the link member and hence the range of angular movement of the link member. It is particularly advantageous if the second valve takes the form of a butterfly valve having an actuating pin offset from the axis of rotation of the valve such that movement of the actuating member about the axis of the valve effects operative movement of the valve between an open and a closed position. Since butterfly valves are essentially balanced devices, they only required a relatively small amount of torque for full range movement and power consumption for operation of the turbocharger assembly is therefore reduced. The link member may alternatively be mounted for translational movement.
Preferably the valves are aerodynamically balanced, that is the gas forces developed on the valves are balanced, for example are disposed equally on either side of the pivot axis in a rotary valve or do not have a resultant component in the slide direction in the case of slide valves. In this way, the forces required to operate the valves are kept small and, more importantly, relatively constant during operation, which has the advantage of reducing the power requirements of an actuator utilised to operate the link member.
The first and second valves are preferably realised as rotary valves whose axes are parallel to each other, the first valve being a swing valve and the second valve a butterfly valve. However, other valves such as barrel valves, ball valves, slide valves, as well as different combinations thereof, are also possible within the scope of the invention, it merely being necessary that the elongated channel be configured so that no movement of the second valve occurs during the first range of movement of the link member whether it be configured for translational or rotational motion, and the appropriate camming action is applied to the actuating member of the second valve to effect operative movement of the second valve between its open and closed positions during the second range of movement of the link member.
In the preferred embodiment, the first valve is a swing valve having a pivot axis about which the link member is mounted for rotation. The swing valve can then be coupled to the movement of the link member very easily by means of an actuating pin attached to the swing valve in a position offset from the pivot axis which engages in a complementary shaped opening in the link member. This has the advantage of simplifying the design of the link member. Alternatively, the axis of the rotation of the link member may be offset from that of the swing valve, in which case the link member has a second elongated channel in which the actuating pin of the first valve engages. Furthermore, by appropriate shaping of the second elongated channel, the link member may be mounted for rotational movement and the first valve may be a slide valve, or the first valve may be a rotary valve and the link member mounted for translation movement.
Preferably, when the link member is in the start position the first valve is closed and the second valve is open. In this way, all exhaust gas from the engine is directed through the turbine in order to maximise the boost pressure of the turbocharger. In the second position the second valve remains fully open and the first valve is partially open so as to allow a portion of the exhaust gas to bypass the turbine. During conditions when the catalytic converter is at its normal operating temperature, the link member will then move from the start position towards the intermediate positions as the engine speed increases and vice versa so as to prevent over pressuring in the turbocharger. In the end position of the link member, the first valve is at its extreme open position whilst the second valve is closed so that all the exhaust gas bypasses the turbine and instead passes directly into the exhaust system and to the catalytic converter. In this way, during cold start conditions the exhaust gas reaches the catalytic converter with minimal heat loss, thereby ensuring that the catalyst reaches it operating temperature in the shortest possible time.
Preferably, when the second valve is in its closed position, a small amount of leakage of exhaust gas past the second valve to the turbine volute still takes occurs. This may be achieved by the second valve not being in a fully closed position when the link member is at its end position or by designing the valve plate of the second valve to permit a small amount of leakage, such by including through openings therein or by making the plate slightly smaller than the cross-sectional dimensions of the turbine volute inlet duct in which it is located. For example, the valve may be a barrel valve having a main through opening which is aligned with the axis of the turbine volute duct when the valve is open to allow gas to pass therethrough as well as a through-opening at right angles thereto which is aligned with the axis if the duct when the valve is closed so as to allow a controlled degree leakage of gas to the turbine. In this way, the turbine will continue to spin, albeit slowly, even when the link member is in its end position, this having the advantage that it helps to prevent turbine seal oil leakage which is known to occur when a pressurised oil supply feeds a non-rotating turbocharger within a vibrating environment and which increases exhaust emissions. It also has the advantage that it helps to prevent bearing damage which can occur in a turbocharger which is constrained from rotation after repeated cyclic vibration. The rate of leakage can also be set as required, for example by varying the size of the hole, to set the desired xe2x80x9ctickoverxe2x80x9d for the turbine.